The present invention relates to an object detection device with a variable sensitivity electric field measurement function and, more particularly, to an entrapment prevention device using the object detection device.
Conventionally, devices have been used to prevent entrapment of the hand, head, or other human body part from being trapped in a vehicle power window device or building automatic door. For example, in power window devices equipped with an entrapment prevention device, when a foreign object is located between the window glass and window frame when the window is closing, the window closing operation is stopped so that the foreign object is not trapped therebetween. Recently, it has been suggested to implement an entrapment detection device based on static capacitance measurements.
JP Patent Publication No. 2001-32628 describes a static capacitance detection unit of a conventional entrapment prevention means. The conventional entrapment prevention means measures the static capacitance formed between the window glass and the sensor electrodes provided in the window frame, and if the measured capacitance exceeds a reference static capacitance, the entrapment of a foreign object is detected. However, in this configuration, as the static capacitance generated when the object is trapped between the window frame and glass is set as the reference static capacitance, the foreign object is only detected when it is actually trapped. In other words, for this device, as the foreign object being in contact with the window glass is the premise for entrapment detection, the accuracy of entrapment detection depends on the reference static capacitance. For this reason, regardless of whether entrapment occurs, the device is unable to appropriately detect entrapment. Furthermore, if the size of the foreign object is rather large, entrapment is only detected after a large force is applied to the foreign object.
JP Patent Publication No. 2010-1637 describes another example of a prior art entrapment prevention device. This device uses a static capacitance sensor to detect an object in a non-contact manner. The static capacitance sensor detects an increase in static capacitance when a foreign object (a person's body part, etc.) is placed between two electrodes. When the varied amount of the capacitance exceeds a fixed value, the device determines that a foreign object is near a closing movable object (a glass window, etc.) and stops the movable object or initiates opening of the movable object. As the foreign object is detected beforehand, entrapment is prevented.
However, the device of Reference 2 may be overly sensitive in the detection of foreign objects. For example, when a window glass is closing due to rain, the static capacitance sensor may be overly sensitive to the rain drops, that is, mistaking the rain drops as foreign objects and stopping the closing of the window and even opening the window. For this reason, maintaining good detection accuracy while preventing undesired movement of the movable object are grounds for improvement for the prior art entrapment prevention device.
It is an object of the present invention to provide an object detection device that has good detection accuracy when a foreign object is near a movable object yet is not overly sensitive.